A honeycomb structure having a wide area is produced by joining honeycomb panels to each other. FIGS. 10A and 10B show an example of the joining method described in Japanese Unexamined Patent Publication (Kokai) No. 62-238072. In FIG. 10A, when a honeycomb panel 1-1 and a honeycomb panel 1-2 are joined to each other, a face sheet 2a-1 is joined to a face sheet 2a-2 and a face sheet 2b-1 is joined to a face sheet 2b-2, but honeycomb cores 3-1 and 3-2 have a small thickness and all of these members are not always positioned for butt welding with each other. Therefor, since the face sheets alone are joined to each other, a problem arises in that a satisfactory bond between the honeycomb panels can not be attained as will be described in detail later.
In FIG. 10B, an insert material 4 having a height larger than the thickness of the honeycomb panels is put between the honeycomb panels, and welding is effected between the face sheets 2a-1 and 2a-2 and between the face sheets 2b-1 and 2b-2 via the insert material 4. In the method shown in FIG. 10B, however, since the face sheets alone are joined to each other without joining of the honeycomb cores, a problem arises in that s satisfactory bond strength can not be attained as will be described in detail later. Further, the protruded insert material 4 causes the joints to bulge, which spoils the appearance of the resultant honeycomb structure. Moreover, it is troublesome to insert the insert material each time joining is to be effected.